Receiver for electrical pulses



July 26,1949 M. M. LEVY 2,476,985

RECEIVER FOR ELECTRICAL PULSES Filed Jan. 16, 1945 5 Sheets-Sheet 1 F/G. /b.

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RECEIVER FOR ELECTRICAL PULSES Filed Jan. 16, 1945 July 26, 1949.

M. M. LEVY RECEIVER FOR ELECTRICAL PULSES 5 Sheets-Sheet 3 Filed Jan.16, 1945 y m Y 7% Wm M n z 1cm A O M M! M y B July 26, 1949. M. M. LEVY2,476,985

RECEIVER FOR ELECTRICAL PULSES Filed Jan. 16, 1945 5 Sheets-Sheet 4 5WWW VVWV W Trace 2O Trace o Ip Zero vemjca/ a Inuenlor NRMWZE MovnE L9HJuly 26, 1949. M. M. LEVY 2,

RECEIVER FOR ELECTRICAL PULSES Filed Jan. 16, 1945 5 Sheets-Sheet 5 /ST.'BE/IM QOUBLE Gil/V 056716 OSCOPE KIM/ L #751? INVENTOR. MAURICE MO/SEAft Y Patented July 26, 1949 RECEIVER FOR ELECTRICAL PULSES MauriceMoi'se Levy, London, England, assignor,

by mesne assignments, to International Standard Electric Corporation,New York, N. Y., a

corporation of Delaware Application January 16, 1945, Serial No. 573,101In Great Britain December 1, 1943 Section 1, Public Law .690, August 8,1946 Patent expires December 1, 1963 3 Claims.

The present invention relates to receivers for electrical signals,particularly pulses and which utilise a cathode ray oscillograph forgiving an indication of the reception of the signals which have atendency to be masked by noise signals.

Receivers of the type specified are particularly applicable, forexample, to radio location equipment for determining the presence ofobjects, for example, such as aircraft, or for determining distance bythe utilisation of the reflection of electrical pulses in the form ofdirect current pulses or in the form of trains of high frequency Waveswhich are detected to produce a direct current pulse which is thenapplied to the cathode ray osci llograph to obtain the requiredindication. The invention is also applicable to other cases utilisingthe reception of an electromagnetic signal wave which requires arelative indication rather than an absolute value indication. Forinstance, in some cases utilising the reflection of electromagneticwaves, it is desired to adjust the frequency of the transmittedcontinuous wave until the wave received, after reflection at an object,has a specific phase relationship, for example, in phase or anti-phase,to the contemporaneous'ly transmitted Wave. The frequency of thetransmitted wave is adjusted until this condition is attained whichcondition is given by an indication on the oscillograph screen.

'In obtaining these indications the electron beam is caused to scan thescreen in one direction, for example, the horizontal or abscissadirection under the control of a time base circuit whose period is equalto the pulse repetition period or other convenient period. The receivedsignal is applied after the necessary treatment, for example,amplification and rectification, to cause the beam to be deflected in adirection perpendicular to the scanning direction, e. the verticalthree-- tion in the example given.

These reflected signals, as will be understood by those skilled in theart, are very weak in intensity or energy level, andare difiicult todetect when mixed with noise which appears on the oscillograph screen asan irregular wavy horizontal line which tends to mask the indication ofthe received signal.

It is the object of this invention to provide arrangements forincreasing the signal to noise ratio of the indication on theoscillograph screen and thus rendering the signal indication moreclearly distinguishable and prominent against the background noiseeffect produced on the screen.

The invention: resides in limiting the vertical or ordinate deflectionofthe electron beam to a. predetermined ordinate value irrespective ofthe instantaneous deflecting force of the received signal above thatpredetermined ordinate value.

Arrangements according to thisinvention comprise ampl-itude limitingdevice which limits the amplitude of the recsivedsignal beforeapplication to the ordinate deflecting unit of the cathode ray tube. Bythis means the deflecting force on the electron beam is maintainedsubstantially constant: during the greater period ofreception of thesignal, with a consequential brighter indication on the screen duringthat portion of the period compared to the brightness during theremaining portion-of the period when the ordinate deflecting force isless than or does not persist for so longas the deflecting force due tothe signal above the predetermined value.

The invention also includes arrangements for increasing the visibilityof indications produced by contrast of the intensity of illumination ofan area with respect to the intensity of illumination of surroundingareas comprising means for scanning said areas, for example, a cathoderay oscillograph screen having an indication produced as hereinbeforespecified, as regards intensity of illumination and means for producingan amplified indication of the variation ofintensity of light along thedirection of scanning.

The principle on which the invention is based is deduced from the'law ofdistribution of the peak amplitudes in the-noise.

The invention will be more clearly understoodf-rom-thefollowingdescription taken in conjunction with the accompanyingdrawings in which Figs. 1 a, 11),- 10, 2a, 2b,- 3, 4 and 5 show curvesused in explaining the invention; v

Fig. 6 shows the circuit arrangements of an embodiment of the invention,given by way of example only; 7

Fig. 7 is a further explanatory curve;

Fig.. Sshows diagrammatically an arrangement for improving. theindication obtained by means ofthe present invention;-

Figs. 9' and 91; show the kind of trace obtained by the improvedarrangement shown in. Fig. 8-; and

Figs. 10 and 11 show, diagrammatically, arrangements according to theinvention.

The noise may be looked upon as the resultant of an infinite number ofpulses of very short duration, the amplitudes being-distributed atrandom. It will: be assumed that these amplitudes follow Gauss law ofdistribution. If: a curve be traced. the ordinates of which representthe peak amplitudes of the noise pulses and the .abscissae theproportion of pulses occurring in a specified time having a givenpeakamplitude, that is the value of the probability f or a noise pulse tohave a given amplitude, a curve suchas the one represented at inFig. 1ais obtained. Pulses having zero amplitude are the most probable and theof the noise amplitude by the relation pulses, this curve may berepresented by the can 1 1M2 Now since the movement of the spot of thecathspot is at. a certain amplitude follows the same kind of law, andthe distribution of the intensity on the screen produced by the movementof the spot follows also the same law. That is to say; the curveC ofFig. 1a represents also the law of noise. This intensity is maximum onthe abscissa line and decreases very quickly at points away from it.

It should be observed that this is really a hypothesis exactly'as Gausslaw of distribution. This hypothesis is most probably correct. However,even if the intensity distribution follows a different kind of law, thegeneral principles of the method explained hereinafter can still beapplied. For simplicity, and as an example, it will be assumed that theconditions obey the laws referred to.

What happens when the signal pulses aremixed with the noise will now bediscussed. Fig. 1c rep- 1 resents the resultant of the noise of Fig.' laand the pulse of Fig. 1b. When a signal pulse appears, its elfect is toincrease the noise deflecting force by an amount equal tothe amplitudeof the signal pulse. Thishas been shown at B in Fig. 1c. The

intensity distribution curve has been traced for 1 a point A where thereis no signal pulse and for l a point B where the signal pulse ispresent. The

intensity "distribution law is the same, but the pulse raises the curveby an amount equal to the 7 pulse amplitude.

The value of the minimum amplitude of the signal pulse which can bevisible can now be found,

that is, the minimum value of the signal noise ratio for which thepresence of a signal pulse may be detected by visual observation. Y

, In Fig. 1c the root mean square noise amplitude (standard deviation instatistics) has been represented by the ordinate Aan. The intensity oflight for this point is nearly 60% of the valuefor point A. Assuming avariation of 40% is nearly the minimum variation which can bedistinguished, one can see that the minimum lift of curve B making thisvariation In visible is, in

the example shown, equal to Aan. Thus, the minpulses is'approximatelyone.

imum signal/noise ratio for visibility of the signal A method by which,much smaller values of signal/noise ratio can be made visible will nowb de c ib d I I a 7 One practical means is to insert an amplitudelimiter in the receiver, at any point before the cathode ray tube, v, V

Theeifect of this limiter. on the intensity distribution on .the cathoderay tube will now be examined. Assume first no pulse signal is re-'ceived, Without the limiter the intensity distribution curve is assumedto be Gauss curve as where k is a constant connected tothe root mean 7 sre valu qua V e H ode ray tube is random the time during which the grepresented at C, in Fig. 1c and Fi 2a. Assumenow the limiter euts allamplitudes above or below a given value, AaL above if AocL is greaterthan the average intensity on this line is equal to the area of theintensity distribution curve occurring above this line. This averageintensity is negligible when AozL is great; and increases gradually whenAozL decreases and becomes sharply maximum when AozL=O (see curve D,Fig. 2a) whereas the amplitude of the noise intensity distribution(curve C) varies very slowly for very small values of AaL. If AaLstarting from a zero value intensity light will not vary, but if alimiter is used the variation will be marked (compare curves C and D,Fig. 2a). This property is used in carrying out the present invention inorder to increase the visibility of indications due to very smallpulses. The principle, is to use an amplitude limiter, preferablyadjusted so that AaL is small or zero, and to observe on the horizontalscanning line produced by the limiter the difference of intensitybetween the points or instants where there is onlynoise and the pointswhere the noise is mixed with a signal pulse. This differenceinintensity is explained as follows.

Fig. 2b represents the average intensity light on the scanning line fortwopoints: one A, where there is no signal pulse, andone,B, where thenoiseis mixed with a signal pulse. The effect of the pulseis to lift thenoise curve of a heighten equal to the amplitude of the pulse. Theaverage intensity on the scanning line at point A is I0, the maximumamplitude of the curve Ib, and at point B it is Ip. The diflerence Io-Ipis pro portional to a when cc is small because the curve near the cuspis approximately constituted by plained; in connection with Fig. 1c.

7 To have an idea of the improvement intro duced by this method somecalculations are neces Let In be the noise light, intensity for anordi-j nate of abscissa Z. The law of distribution intensity of light isgiven by of the where K is a constant connected to the root mean squarevalue 'of In by the relation 95 Let Ib be the intensity of light on thescanning line Z-, when there is a limiter. Then we have where 7 used instatistics problems. There exists numeri- :"0 "and below is if AczL isless than '0 (Fig. 2a,);

I 3 creases by a very small quantity, the noise inthe verticaldistribution of the intensity due to two straight lines. Without thelimiter for very small values of there is. no appreciable variation ofintensity along the sweep line, as already excal-tables of this integraland "from these tables, the curve shown in :Fig. 3 canbe traced. Inthis'figure'thecurve representing In is also shown and to makecomparison easier, the .two curves are. given the same. maximumamplitudes. The root mean square amplitudeoi the noise is representedthe abscissae an as in Fig. 1c. The intensity 112' corresponding to theroot mean square amplitude is also represented. It maybe seen that withcurve ID. the same amplitude 5. can be. obtained with half thedeflection an, that is to say with half the amplitude of the signalpulse which will be necessary with curve In. Thus, the above methodimproves the signal to noise ratio by about 6' dbs.

It. may be noticed that the adjustment of the limiter may be independentof the amplitude of the noise. and signal pulses and a limitersuppressing either the posiiive or the negative part of the signals maybe used. Any amplitude limiter, .for example a biassed. amplifier, canbe used. Many such limiters are well known and practically all are verysimple. Thus the invention. is very easy to embody, even in any existingcathode ray oscillograph indicating apparatus.

The above. theory is only approximate because of the example ofintensity curve. chosen. In practice, the scanning, line produced by thelimiter must be adjusted in order to get the best results.

In the description given hereinbefore, the intensity distribution. lawhas been assumed to be identical to Gauss law. Experiments have shownthat. the phenomenon described" does occur in practice. It isinteresting to notice, however, that this phenomenon is practicallyindependent of the kind of intensity distribution law so long as thelawwhich the intensitydistributiondoes fol low'presents a slow rate ofchange of intensity with distance fromthe sweep line at the point ofmaximum intensity (curve C, Fig. 2a,) That is, the-first derivativeortangen-t at the maximum of curve C is zero, and the curve representingthe law is convex at its maximum rather than cuspshaped like curve D. Acomplete analysis shows that even ifcurve-C is cusp-shaped; thephenomenon occurs for AuL=0 providing that the maximum amplitude ofcurve C is less than the maximum amplitude of curve D.

The case will now be considered of a limiter whose cutoff amplitudevaries progressively from u to +11.

Referring to Fig. 2a, it may be seen that the limiter limits all peaksWhose maximum amplitude is greater than AaLI negative peaks are notlimited. If AocL has a negative value, the limiter can limit either allpeaks above the limiter line (line parallel to the abscissae and. whoseordinates are equal to AaL) or below this line. To trace the symmetricGauss curve of Fig. 2a, it has been assumed that the peaks below thisline are limited. We will examine now the case where all peaks abovethis line or alternatively all peaks below this line are always limitedwhatever may be the sign of AocL.

Assume that all peaks below the limiter line are limited. If AaL=a, theintensity along the limiter line is zero. When the limiter amplitudecomes nearer zero, the intensity along this line increases following alaw represented by Gauss curve (curve C, Fig. 2a, or curve C, Fig. 4).When the limiter amplitude is positive, the intensity still increases,as we assume that all peaks below this line are limited. Finally, at thelimit, when the 6 limiter amplitude is +w, the intensityis maximum. Thelaw is represented by curve C; Fig; 4

With this of limiter, Fig. 2b: becomes as represented in Fig. 5 withthenotations of Fig. 2b), except that the limiter as regards Fig. 5 issuppressing the signal: above the limiter'line instead of' below this: liiie as is the case for'Fig; 2b. The limiter giving the conditions shownin Fig. 5' may be said to give results which arecomplementary to thosegiven by the limiter studied in connection with Figs. 2w and 2b.

All these different kinds of limiters are easy to produce by thoseskilled in the art. A simple circuit is represented-in Fig; 6-. Thenoise mixed with signals is appiied at P on the input grid G of anamplifying val ve V. The plate resistance R, is shuntedby a diode E inseries with a battery F. The plate of valve V is also connected to plateof diode E. The cathode of diode E is connected to battery F. Valve 'V'is prefierably a pentode so that plate voltage variations do not afiectthe plate current of the valve. It is clear that this circuit will actas a limiter oi the type. described above. If the cur-rent of valve V issmall and assuming the H. T. voltage is greater than the voltage of thebattery F, a currentwill; flow across R and diode E to battery F so thatthe voltage across diode E is small; let Is. be this current; thevoltage across resistance'R will start to vary only when the plate.current oi valve V is greater than Is. This explains the limiter actionof the diode. AccL can be ad.- justed' by a convenient choice of thevoltage or battery" F.

Thepotentials across R are appliedto the ver--- tical deflecting platesof a cathode ray oscillograph- 0- and a time base circuit provides thehorizontal defle cti ng. potentials for 0 In order to make theindication obtained the cathode ray oscillograph screenv more visible,

two limiters may be used, for example, two am plifier valves fed inparallel with the signaland noise and eachhaving a limiter connected initsontput, for exampleas shown in- Ti-"ig. 6. A cathode rayoscillograph' having a'doublebeam, that is, two beams or one split beamis used and one limiter is arranged to suppress me lower part of thesignal and noise pattern, so as to produce the trace l with one beam asshown in Fig. 7, whilst the other limiter is arranged to suppress theupper part of the pattern and produce a trace as shown in trace 2, Fig.7, with the other beam as hereinbefore described. Trace I is identicalto the trace shown in Fig. 2b, and trace 2 is identical with the traceshown in Fig. 5. A pulse is shown as occurring between the times t1 andt2 and it will be seen that the intensity of illumination of trace lduring the pulse is weaker whilst in trace 2 the intensity is stronger.The presence and occurrence of the pulse is seen more clearly by thecontrast between the two traces.

Fig. 10 represents, diagrammatically, an arrangement incorporating thisfeature of the invention.

In another alternative arrangement shown diagrammatically in Figs. 8 and11 for making the indication of a received pulse more visible, an opaquescreen [0 covering the face of the oscillograph screen I2 is providedwith a horizontal slit ll located so as to expose only the horizontaltrace of the oscillograph. This slit is also covered and means areprovided for uncovering in a sweeping movement a. small area 7 of theslit. ther disc [3 having slots [4, the disc l3 being rotated by themotor 2|. The disc I3 may be in the form of a Nipkov disc as used toscan a screen in television apparatus or the disc may have a continuousspiral slit in place of the spiral series of holes. By this means onlyone small or elemental area of the trace of the oscillograph I 5 isvisible at a time. The light from this small area is focussed by a lensI6 on to a photoelectric cell IT. The horizontal plates of theoscilloscope l8 are connected to a time base 22 synchronized with disc13, for example, by a mechanical liaison with motor 20, as shown in thedrawing. If the sweeping frequency of this small or elemental area issmall compared to the sweeping frequency of the beam of the oscillographl5, the photoelectric cell current will be proportional to theaverageintensity of the moving elemental area over'the trace. .This current maythen be used to produce the trace of a cathode ray beam of a secondoscillograph l8 after suitable amplification in an amplifier l9, or tooperate a mechanical indicator. In the case of the second oscillograph,the trace 20 will represent the average intensity distribution of lightalong the trace. on the first oscillograph screen and will appear asshown in Fig. 9a in the case of an'original trace such as I in Fig. 7 inwhich a pulse is, indicated by a reduction of the intensity or as inFig. 9b in the case of an original trace such as 2 in Fig. 7 in which a,pulse is indicated by an increase in the intensity.

It will be understood that the arrangements described for increasing thevisibility of apulse indication are'not limited in their application toa system of the kind described in relation to Figs. 1 to 5, but are ofgeneral application for increasing the visibility of indicationsproduced by contrast of the intensity of an area with respect to theintensity of illumination of surrounding areas.

What is claimed is:

1. Arrangements as claimed in claim 3 comprising means for scanning thetrace produced on the oscillograph screen and means for producing anelectric current which varies in accordance with the intensity of lightalong said trace,

Such means may comprise a fur' W M a second cathode ray ioscillograph'whos'e' beam 2. Arrangementsv as claimed in claim 3 com-' prising meansfor successively and cyclically exposing elemental areas of theoscillograph trace, a photo-electric cell, means for focussing the lightfrom said elemental areas on to said cell to produce a current whichvaries in accordance with the intensity of light inthe successiveelemental areas exposed, an amplifier for amplifyingsaid current orvoltage derived therefromand a second cathode ray oscillograph whosebeam is cyclically deflected in one direction in synchronism with thecyclic exposing of the trace,'and in a perpendicular direction by theamplified currents or voltages derived therefrom. 3. A signal indicatingsystem particularlyfor signals whose amplitudes are small compared tothe noise amplitude comprising 'meansfor limiting the amplitudes of saidsignals inthe presence of noise and means to apply clipped signals to anoscilloscope whereby the signals appear on the oscilloscope trace in theform of sections of weaker or stronger illumination, means for scanningsaid oscilloscopic trace sections and means responsive to said scanningmeans and including a second oscilloscope for indicating said weaker'orstronger illumination.

MAURICE MOISE LEVY.

REFERENCES CITED The following references are of record in the file ofthis patent: V NIT STATES PVA'VIENTSV Date Y 2,424,349 Cawein July 22,1947

